This invention relates to integrated circuits, and more particularly, to circuits with asymmetric transistors such as programmable integrated circuits with asymmetric pass transistors and to methods for forming asymmetric transistors.
Logic circuits such as programmable logic circuits are typically constructed from metal-oxide-semiconductor (MOS) transistors. In many logic circuit applications, MOS transistors are used to selectively pass or block passage of logic signals in response to a control signal from a memory element or other source. When used in this way, an MOS transistor is typically referred to as a pass transistor or pass gate.
Conventional logic circuits use n-channel metal-oxide-semiconductor (NMOS) and p-channel metal-oxide-semiconductor (PMOS) transistors. Pass gates are typically formed from NMOS transistors.
Although satisfactory in a variety of applications, NMOS pass gates have difficulty is passing logic ones. As the voltage at the input of a pass gate rises, the gate-to-source voltage Vgs falls and its threshold voltage Vt rises, making the pass gate weaker. Because of this behavior, the rising edges of logic pulses that pass through conventional NMOS pass gates tend to be broadened more than trailing edges. Pass gate performance can be improved somewhat by using an elevated control signal to overdrive the gate of the pass gate, but only at the expense of increased leakage current and a corresponding rise in power consumption. Pass gate performance can also be improved somewhat by using both n-channel and p-channel devices in each pass gate (i.e., forming so-called CMOS pass gates), but only at the expense of increased pass gate area.
It would therefore be desirable to be able to provide improved pass transistors, methods of forming such improved transistors, and integrated circuits such as programmable integrated circuits with improved transistors.